"Dunno," said a fat boy in a back-to-front baseball cap. Stomach bulging out of a grubby red singlet, he shrugged and spat gum on to the grass. He knew as much as every expert I spoke to, from the sociologist at Luton University to the leader of the council, from the local police to community workers and residents old and young. Not everything in life is explicable.

As the ashes blow away and repairmen mend the scars of three days of rioting, the Marsh Farm estate licks its wounds with an air of bewilderment. Why here and not a thousand other places far worse? Riotology is an inexact science. Though for all its manicured appearance, the one crucial ingredient is here. Youth unemployment seeps like a poisonous chemical through its foundations, deprivation beneath the surface corrodes its many face-lifts, years of doing nothing drives young men mad.

A thin 16-year-old boy, with a razored flat top and knotted bangles spun up on his outgrown mountain bike, performed two perfect wheelies before skidding and landing heavily on his backside. Dusting down his baggy ragga trousers, he said: "It's the buzz. Just the big buzz, the biggest!" and grinned. Others nodded enthusiastically. Fun is not factored into sociologists' thinking, and rioting is fun like no other. "I'll never forget it, never! It was great!"

They'll tell their grandchildren about it some day, the three hot nights when Marsh Farm was ablaze. Maybe some of them have great-grandfathers who helped to burn down Luton Town Hall in 1919, angry soldiers home from the war to find nothing fit for heroes. But they had a coherent objective, while now there is only nihilism with a dash of hedonism.

Connoisseurs of miserable housing estates would stroll through Marsh Farm with surprise. Not riots here, surely, among these green fields, scattered with nice houses, gardens front and back? Here is a recreation centre with pool, shops, a library, a community centre, a health centre, church and schools - in short, the ideal London overspill, circa 1967. But they petrol-bombed or bricked out the windows of every public building. (The rustic wooden playground burnt down long ago when the kids set fire to mountains of the local free newspapers they had been paid to deliver.)

Marsh Farm does also have three tombstone tower blocks, but even here efforts have been made - security doors, floodlighting, clean, shiny floors inside, even a 40-place nursery for working mothers: no urine-soaked stairways, needle-strewn alcoves or dog-shit covered balconies.

In the mid-Eighties, you could walk round an estate like this and pick out the new right-to-buy owner-occupiers by their gleaming Regency front doors, carriage lamps and crazy paving. But the mid-Nineties tells a different story. Forty per cent of Marsh Farm's houses are owner-occupied, and you can pick them out now because many of them are the seedy frontages with rotting window frames. These are the home-owners deep in negative equity with not a penny to spend on maintenance. The boarded-up houses are the vacancies of repossession. Luton has the highest negative equity and repossession rate in Britain, and Marsh Farm has its share.

There was a riot here in 1992, but the national press took no interest. Small riots break out randomly in county towns, in shopping centres, and on estates frequently, but press attention to them is almost as random as the events themselves. It takes a cocktail of circumstances to turn a minor skirmish into a full-blown riot: like drink, hot weather, boredom, lots of people, disaffection, press interest - and a trigger.

The trigger this time, according to all observers, was a 13-year-old boy, famous since he could walk, a master of terror whose fat crime file includes violence, theft, vandalism and bullying. He had absconded yet again from a Surrey secure unit and was back on the estate, although his troublesome family had been rehoused elsewhere. The boy was threatening people at the school with a knife when a teacher recognised him and called the police. (As a result, that school was the worst hit later on.) As he sat on a mountain bike among his friends, a swarm of panda cars descended and swept him away. A false rumour that he was badly beaten up and his arm broken was the trigger that started it all. His friends, mostly 13- to 17-year-olds, hoax-called a police car to a non-existent domestic incident in order to ambush it, a version confirmed by police, councillors on the spot and boys themselves. Then the stone-throwing, rubbish fires and car burnings began, but not much damage was done that first night.

What happened next is blamed by observers variously on one or two commercial television crews, on the SWP, on simple wickedness and the euphoria of mayhem. On Thursday morning, the camera crews who had missed the action turned up and began asking the kids what was going to happen next. One was seen egging on children to attack a school, according to Roy Davis, leader of Luton Council, and the local police inspector. On the second day, when the worst damage was done, older young men took over - those arrested were in their late teens and early twenties. Some young people telephoned skinhead friends from Birmingham and London to join in the fun. The SWP, who are out there every Saturday morning selling the Socialist Worker, rushed out an inflammatory anti-police leaflet and everyone was ready for a night of fun. A policeman was stabbed (saved by body armour), 16 boys were arrested, all the public buildings were fire-bombed or stoned: the library, the three schools, the Jubilee Community Centre, part of the shopping centre. Some said 12-year-olds were joyriding all over the place in stolen cars that they later burnt out. A photographer from the Luton News was coshed and a TV crew was knocked to the ground and had their camera stolen.

The MP John Carlisle turned up next morning and had eggs and potatoes thrown at him. After three nights, the police said they had regained control, but the rave organisers Exodus claimed the glory of ending the riots by staging an impromptu rave nearby, which drew 1,500 young people away from the estate on Saturday night and calmed them down.

A week or so later, the damage didn't look too bad. The primary school, which lost several classrooms, had reopened. Kwiksave, the only supermarket on the estate, was having its looted shop front repaired after armloads of bottles and food were swept off the shelves. The loss of the shop caused most fury among residents, who had nowhere else to go. But one middle- aged woman said, "But do you know what some of the young lads did? They delivered looted tins of food to the old folks nearby, bless their cotton socks." Looking for good in the midst of disaster is a common impulse, though others were scornful of this Robin Hood myth. Another resolute optimist tried to find comfort in the fact that the riot was truly multiracial - black, brown and white lobbing petrol bombs together in happy harmony.

The real damage done is to the reputation of the place and its 3,500 inhabitants. One 17-year-old said bleakly, "I'm leaving school soon. I didn't have much chance, but now when I say I'm from Marsh Farm I'll have no chance whatever, will I? We've been seen on nationwide telly."

Everyone was keen to blame a handful of rotten apples, as the crime rate on the estate has been falling and most of the residents I spoke to liked the place, with few applications for a transfer out. Millions have been spent on it since Labour took over four years ago. The council has tried most of the textbook improvements, and that's what rankles. Why here? How dare they? So senseless, mindless, criminal.

The Vicar of Holy Cross, the modern Anglican church beside the shopping centre, expressed brisk and not particularly Christian sentiments about the rioters. "I go up to them and ask them why. 'Nothing to do', they say. What do you mean, nothing to do? There is everything here for you. What do you want? 'Dunno,' they say. 'Not bothered'." He lists their sins: "No sense of self-help. They have no values, no one to set them boundaries." On the Sunday after the riots, he read out the Ten Commandments, but his words fell lamely on the innocent ears of the faithful few.

Some of the young men he is talking about were mooching in a corner of the car park. Nothing, nothing, nothing, they said. Recreation centre? It costs a lot. The free sessions for those on benefits are at a few peculiar random hours. Community centre? They turn up their nose, nothing really in it for them. Jobs? "I don't try any more. I was dead keen when I first left school. Not now. No point." Unemployment on the estate is officially 10 per cent, but among the young it is far higher. They want what everyone wants, a job, a place to live, a car, some money in their pockets, a future. Instead there is only the rolling empty green of the estate, a well-kept no man's land.

The books say tenant participation is the key to success on such estates, so the council gave money to start a Residents' Association. Iris Hume, who runs it, very nearly despairs. Just 10 people and the local policeman turned up to the AGM recently, so it was adjourned. The next meeting fell apart when a hostile wrecker declared he would stop the proceedings and promptly did so. Neighbourly hatred is the fate of many such tenants' groups.

So where is the spirit of community? The police have failed to get Neighbourhood Watch off the ground. The same few people run everything, energetic helpers with the old folks' lunch club, makers of the costumes for the kids' parade in the Marsh Farm Festival next Saturday. Why won't people get off their backsides, volunteer to help, do something, anything, at least express an opinion? The community remains resolutely apathetic and phlegmatic.

In this way, the people of Marsh Farm are much like everyone else. It always seems strange that those who happen to have the misfortune to live on council estates are expected to be full of the kind of community spirit you would be lucky to find in Tunbridge Wells. Volunteering and organising is a minority pastime, as most people want to get on with their own busy and worrisome lives. The prospect of meetings, with agendas, minutes, points of order and probably quarrels, is a hobby only pursued by the unusual. All in all, people here behave very much as you would expect them to in their circumstances, welfare dependency tempered by riot.

We outsiders go in to prod and probe these curious "communities", these bizarre, unnatural experiments in living. No private developer would ever have devised an abuse of space remotely resembling most council estates. We explore the inhabitants as if they are creatures in a sociological zoo. Enough research has been done, we know what makes them better or worse, we know they have become pressure cookers containing the most helpless/feckless/hopeless, as everyone else escapes. But we don't know why some occasionally explode and why, inexplicably, the worst often don't.

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